Sute ishi (捨て石) (sacrifice stone(s)) are stones that are played in order to be taken by your opponent, so as to gain an advantage.
From suteru which means discarded, abandoned or thrown away and ishi which means stone.
It is a big deal, especially in the middle game, to be able to distinguish between key stones, which for strategic reasons are not to be sacrificed (for example important cutting stones), and disposable stones, which will include some kikashi stones but are certainly not limited to those.